Radio Frequency Identification (RFID) technology employs a radio frequency (“RF”) wireless link and ultra-small embedded computer circuitry. RFID technology allows physical objects to be identified and tracked via these wireless “tags”. It functions like a bar code that communicates to the reader automatically without requiring manual line-of-sight scanning or singulation of the objects. RFID promises to radically transform the retail, pharmaceutical, military, and transportation industries.
Several advantages of RFID technology are summarized in Table 1:
TABLE 1Identification without visual contactAble to read/writeAble to store information in tagInformation can be renewed anytimeUnique item identificationCan withstand harsh environmentReusableHigh Flexibility/Value
As shown in FIG. 1, a basic RFID system 100 includes a tag 102, a reader 104, and an optional server 106. The tag 102 includes an integrated circuit (IC) chip and an antenna. The IC chip includes a digital decoder needed to execute the computer commands the tag 102 receives from the tag reader 104. The IC chip also includes a power supply circuit to extract and regulate power from the RF reader; a detector to decode signals from the reader; a back-scattering modulator to send data back to the reader; anti-collision protocol circuits; and at least enough EEPROM memory to store its EPC code.
Communication begins with a reader 104 sending out signals to find the tag 102. When the radio wave hits the tag 102 and the tag 102 recognizes the reader's signal, the reader 104 decodes the data programmed into the tag 102. The information can then be passed to a server 106 for processing, storage, and/or propagation to another computing device. By tagging a variety of items, information about the nature and location of goods can be known instantly and automatically.
The system uses reflected or “backscattered” radio frequency (RF) waves to transmit information from the tag 102 to the reader 104. Since passive (Class-1 and Class-2) tags get all of their power from the reader signal, the tags are only powered when in the beam of the reader 104.
The Auto ID Center EPC-Compliant tag classes are set forth below:
Class-1                Identity tags (RF user programmable, maximum range ˜3 m)        
Class-2                Memory tags (8 bits to 128 Mbits programmable at maximum ˜3 m range)        Security & privacy protection        
Class-3                Battery tags (256 bits to 64 Kb)        Self-Powered Backscatter (internal clock, sensor interface support)        ˜100 meter range        
Class-4                Active tags        Active transmission (permits tag-speaks-first operating modes)        Up to 30,000 meter range        
In RFID systems where passive receivers (i.e., Class-1 tags) are able to capture enough energy from the transmitted RF to power the device, no batteries are necessary. In systems where distance prevents powering a device in this manner, an alternative power source must be used. For these “alternate” systems (also known as active or semi-passive), batteries are the most common form of power. This greatly increases read range, and the reliability of tag reads, because the tag doesn't need power from the reader. Class-3 tags only need a 10 mV signal from the reader in comparison to the 500 mV that a Class-1 tag needs to operate. This 2,500:1 reduction in power requirement permits Class-3 tags to operate out to a distance of 100 meters or more compared with a Class-1 range of only about 3 meters.
It is well known that the performance of dock-door and other RFID reader systems could be improved if the Inventory Session state persistence of the RFID tags could be accurately controlled. In a dock door scenario, a reader at a dock door instructs all tags to wake up, causing them to set their Inventory Status state to an “A-state”.
RFID tags, especially passive tags, have no real state memory more than a few milliseconds. They literally live off of the power from the reader. If the signal from the reader is blocked, the tag dies. Accordingly, persistent nodes have been added to tags. Persistent nodes currently consist of an analog one-shot. Although most of the tag memory is volatile, the persistent nodes are used to remember whether the tag has been counted or not. If power is interrupted, the persistent node typically defaults the tag into a wake A-state after a delay. The reader can then methodically singulate and query the tags and put them back to sleep (“B-state”). The persistent node will indicate whether the tag has been put to sleep for a few milliseconds (ms), thereby keeping the tag from reverting by default to the wake state after a momentary power interruption. However, persistent nodes, which rely on an analog capacitor, have heretofore been found to be very inaccurate, taking up to minutes to cause the reversion.
Another problem is that in many situations, either the tag is moving or the reader is moving, causing new tags to enter and leave the field. If the reader instructs the tags to wake up, and other tags subsequently enter the field, the other tags won't get the wake up signal and so the reader won't know they are there. One solution would be to send out another wake up command, but that would re-wake all of the tags, defeating the purpose. Another proposed solution is to instruct all the tags to go from the A-state to the B-state and count all of the tags, then recount all of the tags while moving them from the B-state to the A-state. By counting twice, the reader should pick up all of the tags. But this is very time consuming and wasteful, as the entire process has to be performed twice.
Thus, there is a great need for RFID tags that can store one or more “persistent” states accurately for long periods of time. The ideal tag would set its Inventory Status state (either the “A-state” or “B-state” corresponding to the “Session” that it is in) based on the most recent command it received from the Reader. The tag should then retain its Inventory Status state (either A or B) for typically 500 ms whether power is available during this interval or not. The tag also should reset the timing of its persistent state every time it receives a valid Select or other valid command appropriate to the state of the tag. Once the 500 ms interval is exceeded, the tag should automatically revert to its “A-state”. While this is a relatively straight-forward design task for battery-powered tags, this has so far proved an impossible goal for passive tags due to the intermittent nature of their power supply.
The disclosed circuit achieves accurate control of persistent nodes with durations of up to several seconds and does so independently of whether the power supply remains steady or is interrupted during that interval.